Scientists currently know of 112 species who’s preferred habitat is the renal sac of a cephalopod. Actually, each species prefers a different kind of cephalopod—Broadclub Cuttlefish, Argentine Shortfin Squid, Giant Pacific Octopus. Conceivably, each cephalopod is chauffeuring its very own and unique urine-loving passengers.

The Dicyemids, or if you are really old school like me the Rhombozoans (literally Latin for spinning top animal), are esoteric animals with a simple body plan and horrifyingly complex reproduction.

Scientific representation of a Rhombozoan cross section

Think of Rhombozoans as a parasitic Reese’s Cup that likes to hang out in your kidney. The chocolaty outside is a layer of skin/feeding cells and the o’ so delicious peanut butter middle is a fun loving center of cells for reproduction. Thus the biological blueprint of Rhombozoan is basically a core of sex covered in a sheath of eating. No organs. No body cavity. I do mean blueprint as each species has a prescribed number of cells from 8 to 40 depending on the species.

The adults attach to the renal sac wall with distinct head region termed a “calotte” (the reference for the spinning top). Once attached, the outer cells of the Rhombozoan do not actually feed on the cephalopod but with use their cilia to move small particles and nutrients in the urine near themselves to feed upon. While the adults are attached, the young swim free to fully explore all the nooks and crannies of the cephalopods renal system or in special cases venture out into the outside world. Allegedly, Rhombozoans do not hurt their molluscan hosts. However if things get crazy and the Rhombozoans start inviting all their friends like it’s a Snoop and Dre party, the cephalopod may experiences some renal “blockage”.

The reproductive cycle is well…ahem…complicated. Sex, no sex, or switch between the two? Some Rhombozoans may start off reproducing asexually but when the cephalopod reaches sexual maturity, then the Rhombozoan decides to switch too. Sometimes love is in the air….or urine.

A single cell runs along the axis of a Rhombozoan, creatively called the axial cell. The axial cell contains little undifferentiated Rhombozoans called axoblasts. These axoblasts can differentiate into just about anything—kaiju, a new car, a pot of gold, hope—but most likely an adult. In asexual adults these axoblasts can give rise to either asexual or sexual forms. The process is long, twisted, and involves multiple steps, like getting anything approved at my university. When sex is preferred, the axoblast becomes an infusorigen, basically the Tootsie Roll Pop of the reproductive phase with an outer layer of eggs and a center of sperm. These inner sperm eventually fertilize the outer ova. The infusorigen larvae actually leave the cephalopod. Where? Nobody knows. I’m guessing Vegas.

The simplicity of the body plan versus the mind-boggling intricacy of reproduction also tells another tale. Most parasites transition through several forms allowing them to move between different hosts and sometimes free living. The classic example is cestode parasites that require time parasitizing copepods, fish, and eventually birds. Some of these parasitic phases become biologically “simple” and may lose their organs, guts, mouths, and any number of other unnecessary parts.

The adults of Rhombozoans are simple and the complicated life cycle of Rhombozoans seems very indicative of a parasite. Yet, the paradox is that Rhombozoans are only known from cephalopods. Unlike other parasites with these specialized forms and complicated lives, Rhombozoans seem to have no other intermediate or final hosts. Maybe at one time Rhombozoans actually did move between hosts. The cephalopods are an ancient group surviving through the extinctions of many marine organisms. Conceivably, at one point Rhombozoans took vacations in the renal systems of a Mosasuar or other giant ocean predator. When these giant predator went extinct, Rhombozoans evolved and stopped touring other renal systems.

Craig McClain is the Assistant Director of Science for the National Evolutionary Synthesis Center, created to facilitate research to address fundamental questions in evolutionary science. He has conducted deep-sea research for 11 years and published over 40 papers in the area. He has participated in dozens of expeditions taking him to the Antarctic and the most remote regions of the Pacific and Atlantic. Craig’s research focuses mainly on marine systems and particularly the biology of body size, biodiversity, and energy flow. He focuses often on deep-sea systems as a natural test of the consequences of energy limitation on biological systems. He is the author and chief editor of Deep-Sea News, a popular deep-sea themed blog, rated the number one ocean blog on the web and winner of numerous awards. Craig’s popular writing has been featured in Cosmos, Science Illustrated, American Scientist, Wired, Mental Floss, and the Open Lab: The Best Science Writing on the Web.